1. Field of the Invention
This invention pertains to electron guns for devices such as cathode ray tubes (CRTs). More particularly, it relates to improved field emission arrays having integral electrodes.
2. Description of Related Art
A cathode ray tube (CRT) and any other device requiring an electron beam normally contains a hot filament to cause thermionic emission from a cathode. There has long been an interest in developing cold cathodes, depending on field emission of electrons, to replace hot cathodes. For low current devices, such as scanning electron microscopes, there are a large number of patents describing field emission electron guns. There are also a large number of patents for field emission flat panel displays where the field emitter has a low duty cycle. For higher current applications, such as TV displays, prior art field emission cathodes, generally based on molybdenum and silicon, have not proven sufficiently robust for commercial applications. Tip damage occurs from ion back scattering caused by the presence of background gases and the tips fail when driven at high current densities.
It has been demonstrated that carbon-based microtip cathodes can be fabricated and used as a replacement for molybdenum- or silicon-based microtip field emission cathodes. It has also been demonstrated that the diamond can be monolithically integrated with gated electrodes in a self-aligned structure, using integrated circuit fabrication techniques (xe2x80x9cAdvanced CVD Diamond Microtip Devices for Extreme Applications,xe2x80x9d Mat. Res. Soc. Symp. Proc., Vol. 509 (1998)).
Much of the work in field emission cathode development was directed to electron sources for use in flat panel displays. U.S. Pat. No. 3,753,022 discloses a miniature directed electron beam source with several deposited layers of insulator and conductor for focusing and deflecting the electron beam. The deposited layers have a column etched through to the point field emission source. The device is fabricated by material deposition techniques. U.S. Pat. No. 4,178,531 discloses a cathode ray tube having a field emission cathode. The cathode comprises a plurality of spaced, pointed protuberances, each protuberance having its own field emission-producing electrode. Focusing electrodes are used to produce a beam. The structure produces a plurality of modulated beams that are projected as a bundle in substantially parallel paths to be focused on and scanned over the screen of a CRT. Manufacture using a photoresist or thermal resist layer is disclosed. U.S. Pat. No. 5,430,347 discloses a cold cathode field emission device having an electrostatic lens as an integral part of the device. The electrostatic lens has an aperture differing in size from the first aperture of the gate electrode. The electrostatic lens system is said to provide an electron beam cross section such that a pixel size of from approximately 2 to 25 microns may be employed. Computer model representations of the side elevation view of prior art electron emitters are shown.
U.S. Pat. No. 5,786,657 proposes a method to minimize the nonuniform influence of surrounding electric potential on an electron beam from field emitters. A hole in the emitting surface and electrodes with suitable potentials are used to minimize beam distortion. A recent paper discusses the use of field emitter electron guns in a CRT. (xe2x80x9cField-Emitter Array Cathode-Ray Tube,xe2x80x9d SID 99 Digest, pp. 1150-1153, 1999) The paper discusses means for decreasing beam diameters by making smaller diameter gates and other adjustments. Also, the problem of limited pixel definition at the periphery of an ellipse-shaped beam is discussed and fabrication and use of segmented or divided focus electrodes to improve beam focus is described.
Space charge, beam deflection, beam size and position, and other factors influence the shape of the beam when a beam passes through electron optics and is focused onto an object. The shape of the beam may also vary with the angle of deflection when the beam is magnetically or electrostatically deflected. Improvement in dynamic beam-shaping methods and apparatus will provide added value for field emitter arrays for use in CRTs or other devices. The dynamic beam shaping method should be widely adaptable to a variety of conditions where the final beam-shape needs improvement, such as when an electron beam is deflected by a magnetic field. The dynamic beam shaping method should allow for the continued adjustment at different deflection angles of the beam.
Apparatus and method are provided for dynamically adjusting the emitted beam shape from a field emission cathode having a gate electrode. The cathode emitter may be carbon-based, but other emitter materials may be used. The gate electrode in an array of field emission sources is independently controlled for each emitter or group of emitters in different areas of the array. Control of voltage on the gate electrode allows emission to be turned off and on or to be adjusted in intensity from different areas. This control allows for dynamic correction of aberrations in the beam by adjusting the emission area and shape in the emitted beam from the cathode array. Control voltages may be supplied from drive circuitry that may be controlled by a microcontroller.